1. Field of the Invention
The present invention relates to a method for preparing a ring-formed body, and a magnetic memory device and a method for manufacturing the same. More particularly, the present invention is concerned with a method for preparing a ring-formed body used for forming a ring-formed magnetic memory element, a magnetic memory device comprising a magnetic memory element comprised of the ring-formed body, and a method for manufacturing the magnetic memory device.
2. Description of Related Art
A magnetic memory device, such as a magnetoresistance effect type random access memory (MRAM), stores information (“0” or “1”) in a memory element utilizing the direction of magnetization of a very small memory cell formed from a magnetic film. As the memory cell, those having various forms including a rectangular form, an elliptic form, a circular disk form, and a ring form have been proposed.
For improving the memory capacity, that is, memory cell density per unit area, as the memory cell becomes even smaller, it is necessary that the gap between the memory cells be narrower. However, when the distance between the memory cells is smaller, a problem arises in that magnetic field leakage occurs from the memory cell comprised of a magnetic material to the adjacent memory cell. Therefore, for increasing the capacity of the magnetic memory device, magnetic field leakage from the memory cell must be reduced.
It is easily understood that a memory cell in a ring body is effective for solving the above problem. The reason for this resides in that the magnetic flux caused by magnetization becomes a reflux magnetic flux and hence does not leak. In addition, when whole of the element (comprised of a variety of magnetic films and non-magnetic multilayer films) including memory cells has a ring form, there is an advantage in that magnetic field leakage does not occur from a magnetic film for reference of the direction of magnetization arranged near the memory cell.
Therefore, a magnetic memory device using a ring-formed memory cell has been proposed (see, for example, Non-patent document 1 and Patent document 1), and an attempt to prepare a very small ring-formed magnetic film is actually made. In the ring-formed memory cell, data is recorded in the form of “0” or “1” depending on the direction of magnetization in the circumferential direction, namely, clockwise or counterclockwise direction.
For realizing a future magnetic memory device having a large capacity, the ring-formed body is required to have an inner diameter and a width, both of which are of a submicron size or less. Further, for suppressing dispersion of the inversion of magnetic field caused in the memory cells, the memory cells must have a consistent form. However, it is difficult to produce with high accuracy and high reproducibility a ring-formed film which is of a size in the order of submicron and very thin, and one which can be practically used has not yet been produced.
On the other hand, in the method for preparing a ring-formed magnetic film recently reported by Nakatani et al. (see, for example, Non-patent document 2), while rotating a substrate, a film is deposited by ion beam sputtering at an oblique direction on the inner wall of a fine hole which is formed in a resist film by electron beam lithography. The resultant ring has an outer diameter of 0.5 μm and a ring width of 0.1 μm, and relatively uniform, very small rings are formed with high reproducibility. In addition, it has been confirmed by magnetic force microscopy (MFM) that the ring has properties as a ring-formed magnetic film.
[Patent Document 1]
U.S. Pat. No. 6,351,410 (FIGS. 4 to 6 appearing at pages 5 to 6)
[Non-patent Document 1]
Jian-Gang Zhu and Youfeng Zheng, “Ultrahigh density vertical magnetoresistive random access memory (invited)”, Journal of Applied Physics, vol. 87, No. 9, May 1, 2000, p. 6,668–6,673
[Non-patent Document 2]
Ryouichi Nakatani, Noritsugu Takahashi, Hana Asoh, Yoshio Kawamura and Masahiko Yamamoto, “Fabrication of Ring Dot Arrays as a Candidate of Memory Cells”, Proceedings of ICMFS 2002 in Kyoto, 2002, p. 178–179
However, in the method for preparing a ring-formed magnetic film recently reported by Nakatani et al., a film is deposited utilizing the shade of the wall of the hole, and therefore diffraction of the flying sputtering particles inevitably occurs to cause a thin film to be deposited on the inside of the ring. In addition, the film is deposited on the inner wall of the hole and therefore, when a multilayer film is formed, the interface between films cannot be parallel to the substrate, thus causing mixing in the side of the multilayer film. For this reason, this method is limited to formation of a ring comprised of a single layer film. Further, in this method, control of the height (thickness) of the ring is difficult and therefore, it is difficult to form with high accuracy a ring-formed body having a thickness in the order of several nm which is required from a practical point of view when the ring-formed body is applied to a magnetic memory device.
A method in which a multilayer film in a circular form is formed by a conventional lithography technique and then a hole is formed in the center portion of the multilayer film to form a ring is theoretically possible. However, positioning for forming a hole in the center portion of a number of very small circular disks is extremely difficult and is not practical.